Various articles made of polyolefins must be sterilized, either by radiation or by autoclaving. For example, it is known to sterilize articles such as syringe barrels and plungers, tubing, surgical clamps, packaging film, tissue culture tubes, fibers for surgical gowns and sheets with high energy radiation. A radiation dosage of 2.5 to 5.0 megarads is usually sufficient to effectively sterilize such shaped articles and the material contained therein. However, polymeric articles exposed to such radiation typically suffer from discoloration and embrittlement, which can render them unfit for their intended use.
The prior art has sought to inhibit such radiation-induced discoloration and/or embrittlement by incorporating various additives to the polymeric composition prior to molding or shaping of the polymer into a useful article. Thus, for example, U.S. Pat. No. 4,110,185 describes incorporating a low molecular weight, preferably not highly viscous, liquid mobilizer, such as a hydrocarbon oil, into a semi-crystalline polypropylene to increase the free volume of the polymer and allow it to retain its flexibility after irradiation. U.S. Pat. No. 4,274,932 describes incorporating the mobilizer in a semi-crystalline polypropylene which has been visbroken to narrow its molecular weight distribution. See also U.S. Pat. No. 4,474,734. However, peroxide visbreaking produces tert-butylalcohol in the polymer, which causes an objectionable odor.
U.S. Pat. No. 5,371,124 to Cooke provides a good summary of the various additives which have been proposed to enhance the radiation resistance of propylene polymer compositions. See also U.S. Pat. No. 4,888,369 to Moore, Jr. However, any additive must be compatible with the other components of the polymer composition, and may cause other problems, including objectionable odor and/or color, processing difficulties, bleeding of the additive from the article over time, etc. See, for example, U.S. Pat. No. 4,710,524, which suggests that the inclusion of a mobilizing additive as described in U.S. Pat. Nos. 4,110,185 and 4,274,932 produces undesirable handling and imprinting problems. Finally, the addition of an oil mobilizer adds significant cost to the polymer product.
Syringe grade material made from polypropylene typically must be peroxide visbroken from a low melt flow rate (MFR) to obtain a narrow molecular weight distribution and must contain oil as a mobilizer to improve the free radical scavenging ability of a hindered amine light stabilizer additive. The polypropylene material typically also contains a sorbitol-based additive as a clarifier.
It is also known to incorporate an elastomeric component into polymeric compositions in order to enhance the mechanical properties of articles made therefrom. For example, U.S. Pat. No. 4,985,479 discloses a stabilized polyolefin composition which is said to possess good weathering resistance and mechanical properties. The polyolefin can include a synthetic rubber copolymer of ethylene and alpha-olefin, such as ethylene propylene copolymer, ethylene butene-1 copolymer, ethylene hexene-1 copolymer, and terpolymers of ethylene, propylene and non-conjugated diene (EPDM). Other examples of stabilized polyolefin compositions having elastomeric components are disclosed in U.S. Pat. Nos. 4,972,009; 4,785,034; and 4,467,061.
However, the addition of such elastomers typically reduces the clarity of the article, which is often undesirable. This problem is particularly acute for products which are manufactured by injection molding processes in comparison to films. Injection molded articles are often up to ten times or more thicker (0.06 inch) than films (0.006 inch thick), and thus any haze will be much more noticeable in the injection molded article than the film. Films are typically produced by biaxial orientation, which tends to improve clarity and radiation resistance. In contrast, injection molding typically causes an undesirable increase in embrittlement.
Yet another requirement for articles which may be sterilized by autoclaving rather than by radiation is a high heat distortion temperature. Such articles preferably have a heat distortion temperature of at least 70.degree. C., and must have a heat distortion temperature of at least 60.degree. C.
Japanese Patent Publication No. 4-142354 (1992) discloses a radiation resistant, visbroken polyolefin packaging film material prepared from a polyolefin resin composition composed of a crystalline propylene-ethylene random copolymer, a linear low density polyethylene, and an ethylene-butene copolymer rubber together with a specific additive comprising specified amounts of a hindered amine compound, an organic phosphorus compound and a sorbitol compound. Haze (cloudiness) is measured according to JIS K 7105, which is a test of film transparency. (Typically, films are from 3 to 5 mils thick, but can be as much as 8 mils thick.) Comparison of Example 2 with Comparative Example 2 shows that the addition of the linear low density polyethylene is essential to improved radiation resistance. However, linear low density polyethylene substantially reduces or eliminates the clarity of injection molded articles at the typical thickness of 40 to 90 mil for such articles. For example, at 40-90 mil thickness, an article would have a haze of about 60-80%, making clarity essentially non-existent. Typically, syringe parts are 45 to 70 mils thick.
An object of the present invention is to provide an embrittlement-resistant polymer composition which does not require a costly visbreaking step or the incorporation of a mobilizing oil additive to impart radiation resistance.
Another object of the present invention is to provide an embrittlement-resistant polymer composition which contains an elastomeric component yet which is substantially transparent.
Another object of the present invention is to provide an embrittlement-resistant polymer composition which can be injection molded into flexible, substantially transparent articles.
Yet another object of the present invention is to provide a radiation- and heat-resistant polymer composition which exhibits a heat distortion temperature of at least 60.degree. C., and preferably greater than 70.degree. C.